There are navigation devices that are capable of navigating a driver to a destination while playing a DVD or CD, e.g., while allowing the driver to watch a DVD or listen to a CD. It is because those navigation devices can access a map data storage medium such as a hard-disk drive (HDD) or DVD-ROM, regardless of playing the DVD or CD. However, if a user wishes to play the DVD or CD with a navigation device, which accesses a DVD-ROM for map data and has only one disk slot, the DVD-ROM (map data storage medium) needs to be drawn out. In such a situation, the navigation device cannot navigate a driver nor display a map, since it can no longer access the map data storage medium, i.e., DVD-ROM that has been drawn out.
A corridor function for a display device is described in Patent Document 1. The corridor function is a function for clipping a part (or all) of map data stored in, for example, a DVD-ROM, storing the clipped part in a memory (e.g., RAM) for the corridor function, and displaying a map based on the clipped part of the map data in the memory for the corridor function.
In many cases, the memory for the corridor function does not have an enough memory space to memorize all of the map data. An amount of the part of the map data is restricted by an amount of the space of the memory for the corridor function. Therefore, it is desirable to find a method to clip the part of the map data effectively.
In a navigation device described in Patent Document 1, the clipped part of the map data extends along a route selected based on a user's operation. Thus, the memory for the corridor function is restricted; in contrast, a route guidance function of the navigation device is required as much as possible not to be restricted. FIG. 13 shows on a map a route 131 selected based on the user's operation. FIG. 14 shows minimum units of the map data overlaid on the map. The minimum units are distinguished from each other by dotted lines in FIG. 14. As shown in FIG. 14, the clipped part 132 extends almost along the route 131.
In this case, however, an alternative route the navigation device can re-retrieve may be similar to the original route 131. The navigation device therefore cannot re-retrieve a possible optimum route that is significantly different from the original route 131. For example, as shown in FIG. 15, the navigation device can use only path D-E-I and N-O-R in calculating an alternative route in place of the original route 131. The alternative route to be calculated will be inevitably almost the same as the original route 131. In addition, the navigation device cannot display a detailed map corresponding to map data which is not included in the memory for the corridor function.                Patent Document 1: JP-H2-151715 A (JP-2663003 B)        